I feel like for 500USD you can definitely get a designer sale bag at Nordstrom Rack or Sak's off 5th that is nice. I've bought many bags for that price range from these places. Or go outlet shopping. It should be a fun experience and if you go in person you can ensure the size is just right! The only bad thing with this is it will truly be hit or miss depending on what is in stock, but you are not against a deadline, right?
One of my favorite classy and minimalist plus not too expensive bag brand is
FURLA. I feel they are quite indestructible and I like their clean look for a school or work type function. (Also material isn't too soft like lambskin which will be tore up if handled more roughly) I have a nice leather crocodile embossed tote which is really functional and I get many compliments on it. It was under 200 on sale. Maybe originally 500, made in Europe.
Otherwise a brand that I know about from a 6 degrees of separation connection (long story but I know the company is pretty dedicated to working with local craftspeople) is
Billy Kirk. I don't own one of their bags (only bc I prefer to see merchandise in person), but I feel like for a bag that gets better with age and that can withstand being manhandled a bit with work/school supplies, these bags would be very useful. A bit more masculine, but well constructed at least and pretty minimalist, such as this
black leather tote or
others might be down your alley.
Lastly,
DKNY is always a place to look for something minimalist but still sleekly feminine & urban. I've had good experience with their bags for durability as well even though my purses were lambskin they still held up pretty well. For you though I'd probably recommend cow's hide if possible since you are looking for a more functional/robust bag.
.
The little trinket that hangs off the front is removable and there is a sneaky pocket underneath the flap at the front for a phone
